One common technique for recovering gold from gold-bearing ore and other gold-bearing materials, such as ore concentrates, involves leaching gold from the ore using a solution having a cyanide salt, typically sodium cyanide. The gold is complexed with cyanide in the leach solution and thereby removed from the ore material. The cyanide complexed gold is then adsorbed onto granules of activated carbon.
In one process, commonly referred to as a carbon-in-pulp process, the gold is first leached into a cyanide solution. The loaded leach solution is then contacted with activated carbon granules to adsorb the cyanide complexed gold. In another method, called the carbon-in-leach method, leaching of gold to form the cyanide complex and adsorbing of the complex onto activated carbon is performed in a single operation. Activated carbon granules are present in the ore slurry when the ore is being leached and cyanide complexed gold is adsorbed onto the activated carbon as the leach progresses. Typically, the activated carbon is moved in a counter-current flow relative to the flow of the ore being leached.
After adsorbing cyanide complexed gold, the loaded carbon granules are separated from the slurry and the cyanide-gold complex is stripped from the loaded carbon into a hot caustic cyanide solution. The gold is then recovered from the strip solution by normal refining techniques, such as by electrowinning of the gold from solution.
Normal cyanidation processes, such as the ones just described, are effective at recovering high percentages, often in excess of 80 or 90 percent, of gold from many gold-bearing ores. There are, however, several gold-bearing ores for which only very low, unsatisfactory gold recoveries are possible using normal cyanidation techniques. These difficult-to-process ores are often referred to as refractory, indicating that they offer unsatisfactory gold recoveries using normal cyanidation techniques. As used herein, a refractory ore is an ore from which less than 80 percent of the gold is recoverable by normal cyanidation processes.
The refractory nature of an ore may be caused by various materials in the ore. For example, gold-bearing ores can be rendered refractory by the presence of significant quantities of organic carbon materials in the ore. Also, gold in some ores, referred to as refractory sulfide ores, is contained in sulfide minerals present in the ore. Gold contained in sulfide minerals is often highly resistant to leaching by a cyanide solution.
Several processes have been proposed for recovering gold from refractory ores. Many of those processes involve pre-processing of the ore prior to subjecting it to cyanidation. For example, it has been proposed that ores having significant organic carbon content may be subjected to a chlorination treatment prior to cyanidation.
With respect to refractory sulfide ores, several processes have been proposed that involve pressure oxidation of the ore at elevated temperature and pressure in an autoclave in the presence of oxygen to decompose sulfide minerals. The ore is thereby rendered more amenable to subsequent cyanidation recovery of gold.
Pressure oxidation, however, is expensive. Substantial energy must be supplied to heat materials to a suitable temperature to initiate the exothermic oxidation reaction. Autoclave equipment must be designed for high temperatures and pressures and must be highly resistant to corrosion.
The cost of consumed chemicals is also a major concern. Oxygen must be supplied to the autoclave typically at high pressure and in a purified form. Also, acid, typically sulfuric acid, is often added to the slurry prior to oxidation. Additional sulfuric acid is also normally formed in the slurry during pressure oxidation as sulfide sulfur in sulfide minerals is oxidized to the sulfate form. Acid exiting the autoclave following pressure oxidation, whether added or generated, is unsuitable for cyanidation, which generally requires a basic pH. Acid in the slurry must, therefore, be neutralized by the addition of a base material, such as lime, prior to cyanidation.
There is, therefore, a considerable need for efficient processes which reduce those energy and chemical costs, while providing for allowing recovery of high percentages of available gold.